Lithographic printing presses use a so-called printing master such as a printing plate that is mounted on a cylinder of the printing press. The master carries a lithographic image on its surface and a print is obtained by applying ink to said image and then transferring the ink from the master onto a receiver material, which is typically paper. In conventional, so-called “wet” lithographic printing, ink as well as an aqueous fountain solution (also called dampening liquid) are supplied to the lithographic image which consists of oleophilic (or hydrophobic, i.e. ink-accepting, water-repelling) areas as well as hydrophilic (or oleophobic, i.e. water-accepting, ink-repelling) areas. In so-called “driographic” printing, the lithographic image consists of ink-accepting and ink-abhesive (ink-repelling) areas and during driographic printing, only ink is supplied to the master.
Printing masters are generally obtained by the so-called computer-to-film method wherein various pre-press steps such as typeface selection, scanning, color separation, screening, trapping, layout and imposition are accomplished digitally and each color selection is transferred to graphic arts film using an image-setter. After processing, the film can be used as a mask for the exposure of an imaging material called plate precursor and after plate processing, a printing plate is obtained which can be used as a master.
In recent years the so-called ‘computer-to-plate’ (CTP) method has gained a lot of interest. This method, also called ‘direct-to-plate’, bypasses the creation of film because the digital document is transferred directly to a plate precursor by means of a so-called plate-setter. A special type of CTP involves the exposure of a plate precursor while being mounted on a plate cylinder of a printing press by means of a plate-setter that is integrated in the press. This method may be called ‘computer-to-press’ and printing presses with an integrated plate-setter are sometimes called digital presses. A review of digital presses is given in the Proceedings of the Imaging Science & Technology's 1997 International Conference on Digital Printing Technologies (Non-Impact Printing 13). Computer-to-press methods have been described in e.g. EP-A 770 495, EP-A 770 496, WO 94001280, EP-A 580 394 and EP-A 774 364. Besides the immediate registration of the plates in a multi-color press, computer-to-press methods offer the additional benefit that the plate does not need to be handled after exposure and processing and, as a result, that it is not necessary to apply a so-called desensitizing coating (gum layer) to protect the developed plate against fingerprints or other contamination.
Especially thermal plates, which are sensitive to heat or infrared light, are widely used in computer-to-plate and computer-to-press methods because of their daylight stability. Such thermal materials preferably comprise a compound that converts absorbed light into heat. The heat, which is generated on image-wise exposure, triggers a (physico-)chemical process, such as ablation, polymerization, insolubilization by cross-linking of a polymer, decomposition, or particle coagulation of a thermoplastic polymer latex, and after optional processing, a lithographic image is obtained. Thermal plate materials, which are often used in computer-to-press methods, are based on heat-induced ablation. A problem associated with ablative plates is the generation of debris that is difficult to remove and may disturb the printing process or may contaminate the exposure optics of the integrated image-setter. As a result, such ablative plates require a processing step for removing the debris from the exposed material.
A non-ablative plate which can be developed with fountain and ink is described in EP-B 770 494. The latter patent specification discloses a method wherein an imaging material comprising an image-recording layer of a hydrophilic binder, a compound capable of converting light to heat and hydrophobic thermoplastic polymer particles, is image-wise exposed, thereby converting the exposed areas into an hydrophobic phase which define the printing areas of the printing master. The press run can be started immediately after exposure without any additional treatment because the layer is developed by interaction with the fountain and ink that are supplied to the cylinder during the press run. So the wet chemical processing of these materials is ‘hidden’ to the user and accomplished during the first runs of the printing press. A problem associated with such non-ablative, hidden-processing materials is the low run length of the plate.
Other thermal plates, e.g. Agfa's Thermostar™ or Electra™ from Kodak Polychrome Graphics, provide a longer run length but require wet processing with alkaline liquids which may damage or contaminate the electronics and optics of the integrated image-setter and other devices of a digital printing press. Also UV-sensitive plates would be very suitable for CTP because of their potential stability in visible or yellow light. In addition, most conventional UV-sensitive, so-called pre-sensitized (“PS”) plates, which are normally used for contact exposure through a film mask, enable a very high run length in addition to other benefits such as high speed, resistance to press chemicals, and better physical resistance, e.g. towards scratching, than typical thermal plates. However, such PS plates also require alkaline processing, which makes them less suitable for on-press processing as required in computer-to-press methods. Such plates are therefore usually developed in a dedicated processing apparatus and then mounted on the press. This requires quite some plate-handling during mounting and registering of the plates and therefore involves a risk of scratches, fingerprints or other damage or contamination of the lithographic image.
So there is a need for a wet processing method which can be performed while the imaging material is mounted in a printing press, without the risk of damaging or contaminating nearby electro-optical components. Such a processing method opens computer-to-press systems to plates that require wet processing but on the other hand are characterized by benefits such as a long run length or daylight stability.